This invention relates to high speed data communications modems, and is particularly useful in a multi-point system in which a central station communicates with a number of remote stations.
It is common practice to make initial adjustments in a modem receiver at the beginning of receipt of a signal burst, to adapt the receiver to the particular channel and transmitter from which it is to receive data. For this purpose each signal burst will typically include a preamble preceding the actual data to be communicated. The preamble represents overhead time that is wasted for data transmission purposes and should be minimized.
In current technology the length of the synchronization preamble is typically 10 msec or less for modems operating at 2400 bps or below, but is 50 msec or more for modems operating at 4800 bps or above. Modems operating at 2400 bps or below do not require an automatic adaptive equalizer in the receiver, whereas high-speed modems do, and the difference in the training times is approximately the time required to initially train the equalizer so that data transmission can begin.
In typical multi-point systems a large number of the return transmissions are relatively short, of the order of a few characters or a few dozen bits, usually simply reporting that the remote station has no data to send at that time. In such systems the data throughput may actually be higher with 2400 bps modems than with 4800, because of the former's shorter training time, even though long transmissions take nearly twice the time at 2400 bps.